"1/3 of the boiling temperature of water" is just as arbitrary as "32". Generally speaking, you get freeze warnings and black ice warnings when necessary inste

"1/3 of the boiling temperature of water" is just as arbitrary as "32". Generally speaking, you get freeze warnings and black ice warnings when necessary instead of checking to see whether or not the current temp is literally below the freezing point of water. I think OP was just saying that it's good enough for a layperson to use day-to-day.